Integrated circuits need to be produced in environments having a clean atmosphere. Significant failure rates in integrated circuits result when particles greater than one tenth the device linewidth are present. As device linewidths shrink, the tolerable particle size will also decrease. Currently 0.7 micron linewidths are common. In the future linewidths are expected to shrink to 0.1 micron or less. Removal of such small particles is extremely difficult as well as costly because the smaller the size of the particles the greater the number of particles that typically are present. There are a number of other situations in which the analysis of particles in the atmosphere would also be useful including monitoring of toxic dumps, spills of hazardous material, monitoring of automobile exhaust or smoke stacks, etc. Consequently control of a particle source is usually more cost effective than removing the particles once they are airborne. Thus means for identifying a potential particle source would be highly desirable.
Particle detection and analysis in clean rooms and gas distribution systems is typically done by real time, also known as on-line, counting of airborne particles with crude selectivity for size, followed by off-line analysis of particles deposited on wafers or other substrates by microscopic or laser scan techniques that provide size and elemental composition. Real time counting provides the rapid response required for monitoring a particle generation event. However, no information is provided to trace the particles to their source. Standard light scattering particle counters can only detect particles which are greater than several hundred angstroms in size. This lower size limit can be reduced by condensing a fluid on the particles. However, the available fluids are undesirable for use in clean rooms or in real time analysis. Off-line analysis can provide the tracking information, but there is no time correlation to note when a set of particles was generated. Off-line particle detection is also limited by particle size. Only particles greater than a tenth of a micron in diameter contain sufficient material for routine compositional analysis. Off-line analysis therefore only provides information about an ensemble of small particles.
Mass spectrometry is an analytical technique used for the accurate determination of molecular weights, the identification of chemical structures, the determination of the composition of mixtures and quantitative elemental analysis. For example, it is possible to determine the structure of molecules based on the fragmentation pattern of the ions formed when the molecule is ionized. An accurate elemental analysis of the molecules requires obtaining precise mass values from a high resolution mass spectrometer. Mass spectrometers operate in high vacuum, so analysis of atmospheric pressure gases requires that nearly all of the gas be pumped away from the analyte prior to ionization.
Real time or on-line particle analysis by mass spectrometry is normally accomplished by sampling particles through a differentially pumped nozzle and impacting the particle beam onto a heated surface. In surface ionization mode, however, ions are emitted and detected from the heated surface as well as from the sample particles, making it difficult to determine the composition and size of the particle. Additionally, not all elements of the sample will form ions, thus causing discrimination in the analysis. More universal detection can be performed by electron impact ionization of neutrals ejected by the particle-surface collision. This method, however, gives fairly extensive fragmentation and much lower ionization yields than surface ionization. Another problem associated with on-line analysis is that each particle yields a burst of ions on the time scale of tens of milliseconds or less. The transient nature of the signal makes it difficult or impossible to obtain a complete mass spectrum with scanning mass analyzers such as the quadrupole or magnetic sector. The consequences of using these analyzers are poor sensitivity and difficulty in performing multi component determinations.
These problems can be reduced by incorporating many features inherent to single-particle analysis by laser microprobe mass spectrometry. Unfortunately, the laser microprobe functions only in an off-line mode since the particle must be mounted on a solid substrate and the laser beam must be aligned to irradiate the particle.
On line particle analysis has been reported in "On-Line Single Particle Analysis by Laser Desorption Mass Spectrometry", Analytical Chemistry, Vol. 63, No. 18, Sep. 15, 1991, pages 2069-2073 which is incorporated here by reference. However, the reported apparatus had problems associated with detecting and analyzing the airborne particles. Additionally the ability to count and size discriminate the particles was not present thus the source of the particles could not be determined.